Inverse design of a nano-photonic wavelength demultiplexer with a deep neural network approach.

In this paper, we propose a pre-trained-combined neural network (PTCN) as a comprehensive solution to the inverse design of an integrated photonic circuit. By utilizing both the initially pre-trained inverse and forward model with a joint training process, our PTCN model shows remarkable tolerance to the quantity and quality of the training data. As a proof of concept demonstration, the inverse design of a wavelength demultiplexer is used to verify the effectiveness of the PTCN model. The correlation coefficient of the prediction by the presented PTCN model remains greater than 0.974 even when the size of training data is decreased to 17%. The experimental results show a good agreement with predictions, and demonstrate a wavelength demultiplexer with an ultra-compact footprint of 2.6×2.6µm2, a high transmission efficiency with a transmission loss of -2dB, a low reflection of -10dB, and low crosstalk around -7dB simultaneously.

Double-pass microwave photonic sensing system based on low-coherence interferometry.

In this Letter, we propose and experimentally demonstrate, to the best of our knowledge, a novel high-performance microwave photonic sensing system employing a reflective double-pass spectrum-slicing sensing scheme, based on low-coherence interferometry in combination with a dispersive medium. The setup is implemented by configuring a double-pass spectrum slicing sensing scheme, which significantly increases the output power level of a low-coherence optical source by approximately 12 dB to compensate for the optical loss of the system. Moreover, since the light passes through the same optical path twice, the conversion efficiency between the applied optical path difference and the dependent radiofrequency (RF) resonance shift is doubled compared to the conventional approaches. It is also possible to realize a very high resolution thanks to the broad bandwidth of the semiconductor optical amplifier (SOA) spectrum. In addition, this SOA-based scheme enables the future realization of a fully integrated sensing system. As an application example, a highly sensitive displacement sensor was investigated, and the experimental results presented a highly linear relationship between the applied OPDs and the RF frequency shifts. The proposed sensing system successfully achieved a high conversion slope of 5.56 GHz/mm and a nearly constant resolution of approximately 124 µm using a Gaussian power density spectrum.

Distributed optical signal processing for microwave photonics subsystems.

We propose and experimentally demonstrate a novel and practical microwave photonic system that is capable of executing cascaded signal processing functions comprising a microwave photonic bandpass filter and a phase shifter, while providing separate and independent control for each function. The experimental results demonstrate a single bandpass microwave photonic filter with a 3-dB bandwidth of 15 MHz and an out-of-band ratio of over 40 dB, together with a simultaneous RF phase tuning control of 0-215° with less than ± 3 dB filter shape variance.

Optical-to-RF phase shift conversion-based microwave photonic phase shifter using a fiber Bragg grating.

A novel microwave photonic phase shifter structure is presented. It is based on the conversion of the optical carrier phase shift into an RF signal phase shift via controlling the carrier wavelength of a single-sideband RF-modulated optical signal into a fiber Bragg grating. The new microwave photonic phase shifter has a simple structure and only requires a single control to shift the RF signal phase. It also has the ability to realize multiple phase shifts. Experimental results demonstrate a continuous 0°-360° phase shift with low amplitude variation of <2 dB and low phase deviation of <5° over a wideband microwave range.

Instantaneous high-resolution multiple-frequency measurement system based on frequency-to-time mapping technique.

A new microwave photonic instantaneous frequency measurement system that can simultaneously measure multiple-frequency signals while achieving very high resolution and wide frequency measurement range is presented. It is based on the frequency-to-time mapping technique implemented using a frequency shifting recirculating delay line loop and a narrowband optical filter realized by the in-fiber stimulated Brillouin scattering effect. Experimental results demonstrate the realization of a multiple-frequency measurement capability over a frequency range of 0.1-20 GHz that can be extended to 90 GHz, and with a measurement resolution of 250 MHz.

Microwave photonic mixer with high spurious-free dynamic range.

A new linearized photonic mixer structure, which can fully eliminate the third-order intermodulation distortion, is presented. It is based on an integrated dual-parallel Mach-Zehnder modulator to which an optimized RF split and an optimized optical phase shift are applied, in series with a Mach-Zehnder modulator driven by the LO. The mixer achieves a very high spurious-free dynamic range performance, it enables essentially infinite isolation between the RF and LO ports, and it has the ability to function over a multioctave frequency range. Experimental results demonstrate a record measured spurious free dynamic range performance of 127 dB·Hz(4/5), which is over 22 dB higher than that of the conventional dual-series Mach-Zehnder modulator-based microwave photonic mixer.

High conversion efficiency microwave photonic mixer based on stimulated Brillouin scattering carrier suppression technique.

A new microwave photonic mixer that can achieve a high conversion efficiency is presented. It is based on using the stimulated Brillouin scattering loss spectrum to suppress the optical carrier at the output of two optical phase modulators driven by the RF signal and the LO, respectively. Experimental results are presented, which demonstrate a high conversion efficiency of 11.3 dB corresponding to over 26 dB improvement compared to the conventional dual-series Mach-Zehnder modulator based microwave photonic mixer, and wide bandwidth of 0.2 to 20 GHz mixing operation.

Shifted dispersion-induced radio-frequency fading in microwave photonic filters using a dual-input Mach-Zehnder electro-optic modulator.

A simple microwave photonic processor structure with single passband response, and widely tunable capability, is demonstrated. It is based on the principle of shifted dispersion-induced radio-frequency (RF) fading by using a dual-input Mach-Zehnder electro-optic modulator (EOM) that is fed from a broadband optical source with unbalanced input fiber lengths into the upper and lower arms of the EOM, in combination with a dispersive medium. This topology consequently produces a spectral response equivalent to the curve of the dispersion-induced RF fading that is shifted from the conventional baseband location to high frequencies. Therefore, an equivalent single passband is formed without the requirement of the conventional tap coefficients. Experimental results verify the structure and demonstrate a continuously tunable microwave filter exhibiting shape invariance and a single passband. In addition, the filter response sidelobe suppression is also significantly improved by applying a Gaussian windowed profile to the broadband optical source.

Distortion-free spectrum sliced microwave photonic signal processor: analysis, design and implementation.

A new switchable microwave photonic filter based on a novel spectrum slicing technique is presented. The processor enables programmable multi-tap generation with general transfer function characteristics and offers tunability, reconfigurabiliy, and switchability. It is based on connecting a dispersion controlled spectrum slicing filter after the modulated bipolar broadband light source, which consequently generates multiple spectrum slices with bipolarity, and compensates dispersion induced RF degradation simultaneously within a single device. A detailed theoretical model for this microwave photonic filter design is presented. Experimental results are presented which verify the model, and demonstrate a 33 bipolar-tap microwave filter with significant reduction of passband attenuations at high frequencies. The RF response improvement of the new microwave photonic filter is investigated, for both an ideal linear group delay line and for the experimental fiber delay line that has second order group delay and the results show that this new structure is effective for RF filters with various free spectral range values and spectrum slice bandwidths. Finally, a switchable bipolar filter that has a square-top bandpass filter response with more than 30 dB stopband attenuation that can be switched on/off via software control is demonstrated.

Single passband microwave photonic filter with wideband tunability and adjustable bandwidth.

A new and simple structure for a single passband microwave photonic filter is presented. It is based on using an electro-optical phase modulator and a tunable optical filter and only requires a single wavelength source and a single photodetector. Experimental results are presented that demonstrate a single passband, flat-top radio-frequency filter response without free spectral range limitations, along with the capability of tuning the center frequency and filter bandwidth independently.

Programmable multiple true-time-delay elements based on a Fourier-domain optical processor.

A new technique to realize an array of multiple true-time-delay elements, which can be independently and continuously tuned, is reported. It is based on a WDM parallel signal processing approach in conjunction with a diffraction-based Fourier-domain optical signal processor. Programmable linear optical phase transfer functions are realized to obtain different electrical true-time delays. The technique can scale to a large number of wideband true-time-delay lines, with continuously tunable programmable delay. Results demonstrate multiple true-time-delay elements with independent tuning control and verify the concept by tuning the free spectral range of a microwave photonic notch filter. To our best knowledge, this is the first demonstration of multiple independently controllable true-time-delay lines for microwave photonic systems.

Single passband microwave photonic filter using continuous-time impulse response.

A single passband microwave photonic signal processor based on continuous time impulse response that has high resolution, multiple-taps and baseband-free response as well as exhibiting a square-top passband and tunability, is presented. The design and synthesis of the frequency response are based on a full systematic model for single passband microwave photonic filters to account for arbitrary spectrum slice shapes, which for the first time investigates the combined effects from both the dispersion-induced carrier suppression effect and the RF decay effect due to the spectrum slice width, to enable the optimum design to be realized by utilizing the carrier suppression effect to improve the filter performance. Experimental results demonstrate a high order microwave filter showing high resolution single passband filtering as well as exhibiting reconfiguration, square-top passband and tunability, for the first time to our best knowledge.

Microwave photonic quadrature filter based on an all-optical programmable Hilbert transformer.

A microwave photonic quadrature filter, new to our knowledge, based on an all-optical Hilbert transformer is presented. It is based on mapping of a Hilbert transform transfer function between the optical and electrical domains, using a programmable Fourier-domain optical processor and high-speed photodiodes. The technique enables the realization of an extremely wide operating bandwidth, tunable programmable bandwidth, and a highly precise amplitude and phase response. Experimental results demonstrate a microwave quadrature filter from 10 to 20 GHz, which achieves an amplitude imbalance of less than ±0.23 dB and a phase imbalance of less than ±0.5°.

Multiple-bipolar-tap tunable spectrum sliced microwave photonic filter.

A spectrum sliced microwave photonic signal processor structure, which is all-fiber based and features simplicity, together with the ability to realize tunability, reconfigurability, bipolar taps, and multiple-tap rf filtering, is presented. It is based on thermally controlled optical slicing filters induced into two linearly chirped fiber Bragg gratings. Experimental results demonstrate the realization of versatile microwave photonic filters with frequency tunable, reconfiguration, and bipolar-tap generation capabilities.

Microwave photonic notch filter based on a dual-Sagnac-loop structure.

A new single-wavelength, coherence-free microwave photonic notch filter is presented. The concept is based on a dual-Sagnac-loop structure that functions with a new principle in which the two loops operate with different free spectral ranges, and which generate noncommensurate taps. It has the ability to generate a narrow notch response and can operate to high frequencies. Experimental results demonstrate a notch filter with a narrow notch width, a flat passband, and high stop-band attenuation of over 40dB.

New multiple-tap, general-response, reconfigurable photonic signal processor.

A new photonic signal processor structure that can realize multiple-taps, with a general response capability, low-noise and widely tunable processor operation, is presented. It is based on a novel concept of employing positive and negative group delay slopes simultaneously by means of a dual-fed chirped fiber Bragg grating, and a new wavelength mapping scheme that enables wavelength re-use. The technique offers scalability, arbitrary responses with both positive and negative taps, tunability, and high frequency operation. Experimental results demonstrate widely tunable filters, with arbitrary bipolar tap generation, no PIIN noise, and with high FSR.